How do establish ac wifi on a Mac?

How do I force an ac connection?

You may not, not without providing a different SSID that you can explicitly select. I don't recommend that and neither does Apple, for technologically sound reasons.

The Mac receives all Wi-Fi signals simultaneously, and will constantly seek the best one available. The algorithm it uses to determine what it considers "best" is not publicly documented.

Yes, my router offers two different names. However, if this is true…

In my case, I don't want different wifi network names. I have some "smart" light switches and security cams that connect on n. If my iMac is on a different network, I can't control the smart devices from my iMac.

Why do you want to force an "ac" connection?

How far away from your router is your iMac? Distance plays a role in which connection your router chooses.

Also, have you tried turning OFF then ON WiFi on your iMac? Then check the connection by left clicking with the Option key pressed on the WiFi icon in your Menu Bar.

I'm afraid I can't be of much help. First, I've never used an NAS drive other than an older Time Capsule which doesn't have ac available. Yes, one would think an ac connection would be faster than an n connection but I have no idea what the expected transfer rate to an NAS drive is on either connection. Transfer rates are affected by many things.

Of course even if you find that connecting to the router using ac is faster, you still can't use it seeing your other devices need an n connection.

Perhaps someone more familiar with NAS drives will chime in.

Maybe there is a lot of overhead involved in a file transfer? Or maybe I am comparing apples and oranges?

Both, really.

Mbps = megabits per second

File sizes are reported in bytes

Then there is the inevitable overhead with file transfers, which has to be bidirectional to communicate data integrity, in addition to the usual processing included with just copying files. All that has to occur within the available bandwidth.

Whilst one would expect it to pick 802.11ac automatically if both your Mac and the WiFi network supports it I would suggest at least temporarily renaming your 5GHz to a different name so you can specifically test it and know which it is using.

It might be that your Mac is deciding the signal strength of your 5GHz network is too poor and therefore prefers the stronger signal of your 2.4GHz network even thought it might be slower.

FYI. 802.11n can run on both 2.4GHz and 5GHz.

FWIW, both 2G and 5G are listed in the list of available WiFi connections for me.

This is about as good as it's going to get, using a new Time Capsule:

The farther away from the wireless router or access point, the slower it will get.